Wednesday, July 09, 2008

I thought my first take on the Ridley Dean bike was a little shabby for my tastes.

So here's a second part to it and some more added thoughts.

Some asked me through comments whether these so called surface textures on the Dean frame are the same deal as dimples on a golf ball?

I will tell you I'm no expert but I do have a BASIC understanding of golf balls.

A golf ball in air is a rotating body. If you give them a critical amount of dimples (too many is counterproductive, too less is simply not enough) on the surface, it'll "catch" a small area of turbulent air around the boundary layer and this turbulent layer causes the air to 'stick' onto the ball. The trailing air will have hence have a longer separation point in its wake which ultimately means less drag and a more long, nice looking streamlined flow.

So is this what the Dean frame is mimicking?

Maybe.

The theory I think is essentially the same here too, but these are no dimples. They are just rough surface textures done here and there. I dont have a microscopic image of what they look like, so I wont call them dimples just yet anyway.

While dimples in golf balls in addition to the Magnus lift effect due to ball backspin have really solid and verified benefits, I'm not really sure about the practicalities for Zipp's wheels or ... the Dean, if any reasonable numbers are out there yet.

But hey, a golf ball is a golf ball. Typical velocities at impact are 60-70 m/s and spin rate is 3000rpm+. I'm not sure whether you want to compare golf balls with Zipp wheels or Ridley's new frame.

So what good is frame texturing to a cyclist? You decide with your money and your head.Ben Atkins of Cycling News has done a good job with getting us laymen some pictures of the bike. I'll spoil it and put some arrows in them too so we all can see whats going on.

FRAME TEXTURE

Theoretical Benefits : Getting turbulent air onto the boundary layer, increasing separation point on the trailing side yada yada.

Here are two places they did this :

Click to zoom

SLOTTED AIRFOIL ON FORKS

In airplanes, control devices called slots on wings are can be fixed or moving.

Remember this? You probably must have seen it through the window seat the last time you flew.

Benefit of slotted airfoils in aircraft : Click this to read.NOTE : I'm not an airplane guy but I understand some of my readers are from the Boeing company. Please improvise in comments if you do read this. What exactly is a slotted wing doing to airflow and talk about the benefits?

Anyway, so similar idea done on Ridley Dean's fork :

So this is supposed to create vacuum around the tire and spokes and decrease drag by 7.5%, and results from tests show that it saves 45watts when ridden at 45 kmph (28mph) ?? It cuts 8X more drag from the fork and spokes area than normal bikes?

Whoa whoa whoa, slow down there. One at a time. Sorry Ridley, can't believe it unless you prove it to us. Take the pen away from the marketing guys.

Give us a graph, a video in the wind tunnel ---- > something we can actually see to believe. For now I like your sharp design, but I'm not buying the specific claims you make about the "fastest bike in the world".

14 comments:

I wonder how many bikes must be sold in order to receive an economic payback for the cost of desig engineering and materials involved in developing this bike. Obviously they have run the numbers but an uninformed guy like me would think that the target market for such a machine would be relatively small.

your linked picture shows, the slotted wing, "energizes" the flow over the entire airfoil (in this case consider it an bicycle fork). By putting the slot in the middle you energize the flow where you would most likely start to see drag and turbulent flow begin. By introducing new energized flow to the fork at the center you eliminate the turbulent flow and drag. When the flow gets energized it continues on laminar flow. after the fork it creates a vacuum for the aft wheel which is rotating into the air rushing over the fork, essentially at 2x's the bikes velocity(similar to drafting). If the fork can keep the air laminar you have a better chance of creating a vacuum for the rotating wheel to run in to, instead of the rotating wheel slamming into the air for very high drag.

One question? is it common for them to put the brakes on the aft side of the forks for TT bikes? That is very cool.

As for return on investment, for that particular model, I doubt they will see one. But over their whole line-up, having a TDF winner sells a ton of bikes.

Now for the actual benefits of all these gimmicks. While most of these "tricks" may have some benefit, I doubt the numbers are as high as they claim. But the psychological edge they give a bike racers (or take away from a rivals) often fragile psyche, can be a huge boost! Many times, simply believing that you have (or don't have) the fastest bike is enough to make the engine all that much more powerfull.

Russ and Tim : Thanks for the theory. Yeah, I have noticed front brakes mounted near the bottom bracket before in a big race. Can't think of the name of the bike .... maybe Cervelo. Yeah its cool, and you can see the extreme to which you could go to be aerodynamic.

Gunnar : Good insight. For something exhausting like the Tour, any additional motivation even though how small it is can really make a difference I guess.

...assuming that there is a correlation between aero- & hydro-dynamics, let's not forget the coating developed by 3-m for the "america's cup" boats, a number of years ago...the concept was, using russ's term, to 'energize' the flow & thus lessen the drag or pull of the water on the boats surface...

...& we're all aware of the exponential increase in drag as velocity is amplified...water or air...

...i don't recall if the 'surfacing' was dimpled, beaded or scored in a particular pattern but it worked...

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...ron...you do realize, don't you, that the 'cute, little disneyesque' diagram you've used to explain airfoil slots is that of the 'me-163 komet', right ???...it was one of "hitler's secret 'last stand' weapons", powered by a rocket motor & capable of flying at 550mph, which was faster than any fighter plane of that era...highly maneuverable although it's powered flight was limited to about 7 1/2 min, the plane had the capability to act as a glider at that point...

...the me-163 would take off on a two wheeled 'dolly' that was jettisoned as it got up to speed & would land on the skid that is pictured below the plane, in your diagram...

...i apologize if i'm creating a redundancy but the komet is a good example because the slots were highly effective, by all reports...

I worked just across the road from where the NZ Americas Cup Boat was maintained for the first cup defence which they won and it had a special surface coating as well. It looked like rough finish paint from where we were allowed to look. As for the behind the front fork brake, John Cob says this doesn't seem to make any difference. See his info at Blackwell http://www.blackwellresearch.com/articles.htm Also, if you look at one of those Oval A 700 or Tektro R725 brakes you will notice that the mounting bracket actually sticks out below the fork base acting like a little dam to any air coming across the wheel and under the fork crown. This is why Specialized have worked hard on this area on the new Transition in this area to flow this air onto the down tube.